In particular, in the case of structural elements situated on the outside of an aircraft, precautionary measures against lightning strikes must generally be taken.
In the event of a lightning strike, first a dart leader plasma channel is formed (streamer-leader formation) and this is followed by a high current discharge (lightning discharge/return stroke). The high current discharge process is connected to a high current flow and introduction of heat into the directly affected CFRP structure. The lightning strike-induced current load associated with the individual zones of an aircraft is specified, e.g., in the EUROCAE ED-84 standard. This does not differentiate between CFRP structures and metallic structures.
Since the thermal conductivity and electrical conductivity of CFRP is significantly lower than that of metals, CFRP structures must be given greater protection than metals. As a standard lighting protection measure, an expanded copper film is applied to the CFRP structure as a lightning protection layer, which serves to conduct away the electric current and the heat. Due to the presence of doubly curved structures, the use of expanded metal foil has been found to be necessary.
It has been found that, in particular in the case of lacquered CFRP structures, lightning strike-induced mechanical damage is substantially due to the lightning protection layer exploding. A non-conductive, dielectric lacquer of this kind is applied to the outside of the structure comprising the metal foil, in order to protect against environmental influences. This explosion of the lightning protection layer comprises an explosion of the copper itself, and of the epoxy resin matrix, by means of which the expanded copper foil is laminated onto the CFRP structure. In the case where a lightning protection layer is not of a sufficient size, the strike current penetrates into the CFRP structure and subsequently the epoxy resin matrix of the carbon fiber composite structure explodes. The explosion of the lightning protection and/or the epoxy resin matrix is caused by the direct heat of the plasma channel and the Joule heat of the lightning protection.
Due to the physical properties of insulating, dielectric layer systems, a concentration or constriction of the plasma channel occurs at the base of the plasma channel during a lightning strike. Insulating, dielectric layer systems also prevent the base of the lightning channel from uniformly sliding over the surface of the structure and thus prevent an even distribution of the heat load and current load over a larger surface area of the structure. The plasma channel can thus remain longer at the point of the lightning strike and thereby cause greater damage to the structure underneath.
To improve the lightning protection it has been proposed that electrically insulating layers be provided under the outer lacquer, but above the lightning protection layer. It has further been proposed that the CFRP structure be protected by means of electrically insulating layers, such as layers comprising glass fibers, arranged underneath the lightning protection layer. However, measures of this type showed little effect or even sometimes increased the mechanical damage in the CFRP structure. By way of example, printed prior art in the field of wind turbines, and in particular in relation to the rotor blades thereof, is known from DE 10 2010 017 62 A1. For lightning protection, the rotor blade comprises, at least in part, a fiber composite plastics material, having solid metal inserts, but not metal fibers, embedded in the matrix of the fiber composite plastics material.
Furthermore, measures have been proposed for greatly increasing the electrical conductivity of the lacquer layer and/or the CFRP structure. However, this has led to no or at most negligible effects, since improved conduction of the lightning would only occur at such a conductivity of CFRP, which would correspond to the conductivity of metals such as copper or aluminum and thus is practically impossible to implement.